The Powerglide transmission was one of the first automatic transmissions developed by General Motors. Although General Motors phased it out in 1973, the basic design is still used today, especially in niche automotive applications, including in automobile power trains designed for racing. The Powerglide transmission has remained popular for racing due, in part, to the strength, durability, and simplicity of the design. Indicative of the Powerglide's continued popularity, it is possible to build an entire Powerglide transmission from aftermarket parts, and a cottage industry has developed around improving the performance of the Powerglide transmission for racing applications. Because of this history, entire transmissions as well as complete replacement parts built to original equipment (“OE”) dimensions and specifications continue to be readily available in the market. Thus, for the sake of clarity, “OE” as used herein refers not only to transmissions and parts manufactured by the original manufacturer but also to any aftermarket transmissions or parts conforming to the OE dimensions and specifications. Similarly, “Powerglide,” “OE Powerglide” and “GM Powerglide” as used herein refer not only to Powerglide transmissions and parts originally manufactured by General Motors but also to any aftermarket transmissions or parts conforming to the OE dimensions and specifications.
When GM Powerglide transmissions are used in drag racing and other high performance applications, transmission line pressure is typically increased for better clutch holding power and quicker shifts. Powerglide transmissions have three friction apply elements: two multidisc wet clutches (High clutch and Reverse Clutch) for high range (also referred to as second gear or high gear) and reverse, respectively, and one double wrap band (Low Band) for low range (also referred as first gear or low gear). Table 1, below, shows which friction apply element is engaged for each gear and shows that only one friction apply element is applied at a time.
TABLE 1Low BandHigh ClutchReverse ClutchPark/NeutraloffoffoffReverseoffoffappliedlow range appliedoffoff(1st gear/low gear)high range offappliedoff(2nd gear/high gear)
FIGS. 1 and 2 are cross sectional views of a prior art Powerglide low range servo assembly 100, with FIG. 1 showing the servo assembly applied and FIG. 2 showing it released. Servo assembly 100 includes a servo piston 102 attached to a servo pin 104, which are both slidably disposed in a transmission case 106. Transmission case 106 includes an apply oil passageway 108 for routing pressurized apply oil to an apply side 110 of servo piston 102 and a release oil passageway 112 for routing pressurized release oil to a release side 114 of the servo piston. As used herein “apply oil” and “release oil” both refer to pressurized transmission fluid, with the terms apply oil and release oil referring to transmission fluid that is being routed to a servo for either applying or releasing the servo. “Line pressure” refers to the pressure of the transmission fluid. Thus, an increase in line pressure results in an increase in both the apply and release oil pressures. Servo assembly 100 also includes two release springs 116, 118 for biasing servo piston 102 and servo pin 104 to the release position (FIG. 2) and a cover 120 for sealing the servo assembly 100 within transmission case 106, attached to the transmission case by bolts 122 (only one of three shown).
In Park, Reverse, and Neutral, servo assembly 100 is in the released position (FIG. 2) and the low band (not shown) is released. To engage low range, pressurized apply oil is directed to apply side 110 of servo piston 102 via apply oil passageway 108, causing the servo piston and servo pin 104 to move from a released position (FIG. 2) to an applied position (FIG. 1), thereby engaging the low band on the high clutch drum (not illustrated). When the transmission is shifted from low range to high range, the low band is released by directing pressurized release oil to the release side 114 of servo piston 102 via release oil passageway 112, causing the piston to retract back to the release position (FIG. 2). In both low range and high range, pressurized apply oil is present and acting on the apply side 110 of servo piston 102 and additional pressurized fluid (release oil) is added to the release side 114, thereby counteracting the apply oil force and allowing release springs 116, 118 to move servo piston 102 to the release position (FIG. 2).
The release side 114 has less surface area than the apply side 110 due to the cross sectional area of servo pin 104. Release springs 116, 118 are sized to provide sufficient force to overcome the force differential caused by this area differential between the release side 114 and apply side 110 of servo piston 102. The spring force of release springs 116, 118 is optimized to provide sufficient force to move the servo piston when the release oil is applied while also not being over-sized to thereby minimize the magnitude of apply oil pressure required to overcome the spring force when servo assembly is applied to engage the band in low range.
When line pressure is increased for high performance applications, both the apply oil and release oil pressure increase, which can cause the low band to drag in high gear due to the area differential between the release side 114 and apply side 110, resulting in a larger force imbalance, which the OE release springs 116, 118 were not designed for.